1. Field of the Invention
The present invention relates to a method of controlling the implant dosage and the pressure compensation factor in situ. More particularly, the present invention relates a method of controlling the in-situ pressure compensation factor through a residual gas analyzer (RGA) so that the concentration of ions implanted into a wafer can be accurately controlled.
2. Description of the Related Art
Ion implantation is commonly adopted technique for implanting dopants into a wafer or specified regions of the wafer in the process of fabricating semiconductor devices. With the reduction in the size of semiconductor devices and an increase in the level of integration, the amount of dopants implanted into designated areas of the wafer must be meticulously controlled. One major factor affecting the concentration of dopants inside an implanted wafer is the degree of vacuum within the processing chamber of an ion implant station. In general, the degree of vacuum inside an ion implant station will deteriorate in proportion to the length of application time. The reason is that the absorbability of the cooling pump for withdrawing air from a high vacuum chamber of the ion implant station will deteriorate with long-term operation. Furthermore, residual gases inside the vacuum chamber will reduce the degree of vacuum and some of the residual gases will react with a portion of the ion beam in a charge exchange reaction. Ultimately, the actual charging state of the ion beam may differ from the preset parametric value. In other words, if a portion of the ions is neutralized to atoms as a result of a charge exchange reaction between the residual gases and the ion beam, the current meter inside the ion implant station will not account for the neutralization. Thus, the current value registered by the current meter inside the ion implant station will be lower than the actual ion beam. Conversely, if the charge exchange reaction between the residual gases and the ion beam has the tendency to strip electrons from the ion beam, the current value registered by the current meter inside the ion implant station will be higher than the actual ion beam.
To resolve the aforementioned problem, the concept of a pressure compensation factor has been introduced. Through the presetting of a pressure compensation factor, the inaccuracy of implant dosage due to a deterioration of the degree of vacuum in an ion implant station is compensated. To determine the pressure compensation factors in the conventional technique, a series of ion implants on a wafer coated with a photoresist layer is performed using different pressure compensation factors and then the sheet resistance of the wafer is measured. Therefore, a linear relation between the pressure compensation factor of the photoresist-coated wafer and the sheet resistance is obtained. In addition, a series of ion implants on a blank wafer is performed using different pressure compensation factors and the sheet resistance of the blank wafer is measured. Thus, a linear relation between the pressure compensation factor of the blank wafer and the sheet resistance is obtained. The cross over point between the two aforementioned linear relations is the proposed setting of the pressure compensation factor.
However, the aforementioned method of determining the pressure compensation factor is based on operations in an ion implant station capable of producing a high degree of vacuum and having a high degree of cleanliness. Therefore, the pressure compensation factor obtained from an ion implant station will be highly inaccurate when the degree of vacuum inside the processing chamber drops or there are variations in other factors. As a result, the ion implant dosage cannot be controlled accurately.